How to get events using multiple custom meta fields?

You can use your current code to create the query and then in your loop run a check before displaying the events something like

<?php 
query_posts('meta_key' => 'end_date_value', 'meta_compare' => '>', 'meta_value' => $todaysDate);

 while ( have_posts() ) : the_post();
   $post_custom = get_post_custom($post_id);
   if ($startDate < $post_custom['start_date_value']){
    //your loop here
   }  
}

?>

keep in mind that this is not well coded way to do this ,I would probably create a better sql query or use “posts_where” hook.

but its a start